On the whole, a good movie, but it makes two major mistakes that keep it from continuing the "each X-Men movie is better than the previous one" trend.
First, they give Halle Berry more screen time. This is problematic, because she gives every line about the same exact reading, which is to say a disinterested, vaguely snappish monotone. She has no screen presence at all; at any given moment, you'll be more likely to watch for background cameos by minor X-characters than you will be to pay attention to whatever she's saying. And given that for one reason or another, Scott, Jean, Xavier, and Rogue don't get much screen time, she has to carry about half the movie. And that's 51% more than she can successfully lift.
The other is (not a spoiler, because it was in the trailers) Jean's back from the dead. This is a story arc that deserves its own movie, but they've decided to try to fit it in at the same time as the "mutant cure" arc that forms the story's A-plot. Which means that neither one has enough time to really work, and further means that the work of establishing the new X-characters (because as previously mentioned, Scott, Jean, Xavier and Rogue are all, for one reason or another, absent for much of the movie) goes unfinished. Beast is nice enough (although the make-up is lousy), but you never get a real sense of who he is. Kitty and Colossus remain cameo-level characters even though they're supposed to now be first-team X-Men. This hurts the movie in all sorts of little knock-on ways. Really, Jean should have come back at the end, and they should have had enough confidence in the franchise to believe that they would do a fourth.
But that's the bad stuff. The good stuff? Everything else.